General oil-based ballpoint pens that have been used have inks employing as solvents ethylene glycol monophenyl ether as a low-boiling solvent, and higher alcohols represented by benzyl alcohol and the like, using dyes soluble in these solvents as colorants, and adjusted to have a viscosity of 6,000 to 20,000 mPa·s (20° C.). In addition, these general oil-based ballpoint pens are designed such that the ink consumption per 100 m of writing duration is, for example, about 10 mg for φ0.5 mm of diameter of a writing point ball, about 20 mg for φ0.7 mm, and about 30 mg for φ1.0 mm in view of strike-through of handwriting, handwriting drying characteristics, ink dangling from a writing point portion and the like. That is, for the ink consumption of a general oil-based ballpoint pen, the writing point portion is designed such that where the ball diameter is X (mm) and the ink consumption per 100 m of writing duration is Y (mg), the requirement of Y≦30X is satisfied, and the requirement of Y≦35X is satisfied even if the ink consumption is set to a higher level. For conventional inks, if the ink consumption is greater than this, there arise problems such as strike-through of handwriting, poor handwriting drying characteristics and ink dangling.
Ink compositions represented by the ink viscosity of the oil-based ballpoint pens described above are all designed in view of the functionality of ballpoint pens, and thus have inevitability. That is, since the ink consumption cannot be increased, a very high concentration of colorant is inevitably employed for maintaining a certain level of handwriting density, and solvent systems are thus limited for maintaining stability of the high concentration of colorant. Since such inks are employed, there are limitations on ink properties and designs of writing point portions as described above.
In this way, the conventional oil-based ballpoint pen has an ink viscosity of 6,000 to 20,000 mPa·s (20° C.), and therefore has rotational resistance of a ball at the time when writing is performed, and does not necessarily have a reduced writing pressure and satisfactory writing feeling. This accounts for the fact that an oil-based ballpoint pen is inferior in writing feeling to a water-based ballpoint pen.
As techniques for solving the problems, inks having an increased viscosity at the time when writing is not performed, and having a reduced viscosity due to ball rotation at the time when writing is performed, as pseudo-plasticity in ink viscosity, are described in Japanese Patent Laid-Open No. 6-313143, Japanese Patent Laid-Open No. 6-313144, Japanese Patent Laid-Open No. 7-196972 and Japanese Patent Laid-Open No. 9-48941.
However, even if the pseudo-plasticity in ink viscosity is imparted to the ink to satisfy both the dangling performance and writing feeling at the same time, limitations on the ink consumption are not eliminated, and thus the problem such that the oil-based ballpoint pen has faint handwriting compared to the water-based ballpoint pen is not solved.
On the other hand, if a dye is employed as a colorant, there arise problems in alcohol resistance and light resistance of handwriting, and studies have been made in a variety of ways. They include employment of pigments disclosed in, for example, Japanese Patent Laid-Open No. 2000-212496, Japanese Patent Laid-Open No. 2001-271017, Japanese Patent Laid-Open No. 2001-279154 and the like. Employment of a pigment in the oil-based ballpoint pen, including employment of a pigment alone and in combination with a dye, has been considered since long ago, and may be well known, but the oil-based ballpoint pen should employ a very high concentration of colorant for maintaining a certain level of handwriting density as described above, and it is thus difficult to keep a uniform state if a pigment insoluble in a solvent is employed.
Furthermore, efficacy in improving the dangling performance by partly replacing a dye with a pigment as a colorant is described in Japanese Patent Laid-Open No. 2001-271018 and Japanese Patent Laid-Open No. 2001-294796, but there is a disadvantage that stability with time is not improved when a pigment is employed due to the problem described above.
Japanese Patent KOHYO Laid-Open No. 9-503819 describes inks with water added thereto or with no water added thereto, but does not so much as indicate that the ink is a solvent solution, which is a water-in-oil-type emulsion.
Employment of a pigment as a colorant is effective means in improving handwriting fastness properties as described above. Furthermore, the applicant of this patent application has disclosed an ink having the improved writing feeling and handwriting density compared to conventional oil-based ballpoint pens by dispersing water in a solvent, in Japanese Patent Laid-Open No. 10-158568, entitled “water-in-oil dispersion ink”, and the dye is a very effective essential component in the sense that the functionality of the oil-based ballpoint pen is satisfied as described above, but has a problem such that the ink included in the ballpoint pen is poor in color visibility in terms of outward appearance due also to introduction of the dye in a high concentration in the ink. For example, a red color does not provide a red color but provides a blackish color in terms of the sense of vision. In this respect, employment of a pigment can ensure adequate outward visibility.
However, employment of a pigment has a very serious problem in stability of the ink with time. The ink for oil-based ballpoint pens has a very high concentration of solids as described above, and particularly the ink in a writing point portion has a small volume and is exposed to various environments. Furthermore, solvent systems are limited for satisfying the functionality of the ballpoint pen, and there are many problems to be solved for maintaining uniform dispersion of a pigment. A wide range of pigments are employed in inks for water-based ballpoint pens but in the case of oil-based ballpoint pens, difficulties concerning employment of a pigment are significantly increased due to the fact that the oil-based ballpoint pen is designed so that an ink channel of a writing point portion is extremely reduced in width, and that the content of solids such as colorant components as well as thickener components is considerably high.
Furthermore, as generally known, employment of a pigment has problems such that the writing tactility is degraded due to a hindrance to ball rotation at the time when writing is performed because of the dispersion system, a ball seat in the writing point portion is easily abraded due to the rotation of a ball, and so on.
The inventor sufficiently analyzed the above problems arising when a pigment is contained in an ink to find countermeasures, and simultaneously solved problems as to the writing tactility and handwriting density, which are inevitable problems in the oil-based ballpoint pen.
The present invention provides an ink for oil-based ballpoint pens having satisfactory handwriting fastness properties, sufficiently improved outward color visibility, satisfactory writing tactility, a satisfactory writing density and improved stability with time, and an oil-based ballpoint pen having the ink.